Field of the Invention
The present invention relates to a technique for sharpening an image produced by image capturing.
Description of the Related Art
Image capturing through an optical system provides a captured image degraded with spread (blur) of a point image according to a point spread function (PSF) of the optical system.
On the other hand, an unsharp masking process is known that applies an unsharp mask to an input image as a captured image to produce an unsharpened image and that sharpens the input image by using a correction component obtained by adding or subtracting a difference between the unsharpened image and the input image to or from the input image. Such an unsharp masking process further sharpens an image area of the input image where the difference from the unsharpened image is larger.
However, a conventional unsharp masking process using a rotationally symmetric filter as an unsharp mask makes it difficult to sharpen a blurred image degraded due to a PSF having a complex shape such as asymmetric aberration and sagittal halo. That is, correcting a large aberration generated in an azimuth direction causes an undershoot in another azimuth direction in which a small aberration is generated, and reducing the undershoot makes it impossible to sufficiently correct the large aberration.
Japanese Patent Laid-Open No. 2010-081263 discloses a method that applies a one-dimensional asymmetric filter to a pixel signal row arranged in an image height direction (that is, in a meridional azimuth direction) to reduce an influence of a PSF.
Furthermore, in astrophysics, a Moffat function is used to fit a photometered astronomical body.
Sebastian Berisha, James Nagy, Robert J. Plemmons “Estimation of Atmospheric PSF Parameters for Hyperspectral Imaging” discloses a method of modeling a PSF by using an Elliptical Moffat function produced by ovalizing the Moffat function. Fitting a PSF of an optical system using such a function enabling approximating the PSF using coefficients.
However, the method disclosed in Japanese Patent Laid-Open No. 2010-081263 only takes account of an asymmetry in the image height direction and uses a one-dimensional correction filter, which cannot improve asymmetries in directions other than the image height direction. Furthermore, this method adjusts the asymmetry of the correction filter by the number of minus tap coefficients, so that its asymmetry in the image height direction does not meet blur caused by the PSF of the optical system, which cannot sufficiently sharpening the pixel signal row. Moreover, the Elliptical Moffat function disclosed in Sebastian Berisha, James Nagy, Robert J. Plemmons “Estimation of Atmospheric PSF Parameters for Hyperspectral Imaging” cannot reproduce complex shapes such as asymmetric aberration and sagittal halo of optical systems.